remove 5ttgen as not ported, and deprecated anyway
remove 5ttgen as not ported, and deprecated anyway

as discussed in #400. Not sure how 5ttgen ended up on this branch anyway
- I have a feeling someone's been doing some cherry-picking from
  master...

MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3